The present invention relates generally to a hologram and a holographic viewing device incorporating it, and more particularly to a holographic viewing device with a built-in computer-generated hologram set up in the form of a transmission type Fourier transform hologram.
Patent Publication 1 comes up with holographic eyeglasses constructed as shown in the perspective view of FIG. 16(a). More specifically, two transmission type holograms 2 and 3 are fitted in a binocular framework of an eyeglass frame 1. As a scene containing light sources 4, 5, 6 and 7, each of small area, as shown in FIG. 16(b), is viewed with the eyeglasses using the transmission type holograms 2 and 3, for instance, it looks as shown in FIG. 16(c). In other words, it looks as if the small-area light sources 4, 5, 6 and 7 in the actual scene of FIG. 16(b) were replaced with pre-selected words “NOEL” 8, 9, and 11, respectively. For the transmission holograms 2 and 3 having such features, Fourier transform (Fraunhofer) holograms having the aforesaid words “NOEL” are used in the form of computer-generated holograms.
A little more explanation is given of the transmission holograms 2 and 3. Suppose now that transmission type hologram 21 stands for the transmission type holograms 2 and 3, and a (small-area) point light source 23 is located behind the transmission type hologram 21, as shown in FIG. 14(a). Then, as the point light source 23 is viewed with an eye 22 through the transmission type hologram 21, one image is reconstructed corresponding to the point light source 23 behind that transmission type hologram 21. FIG. 14(b) is a plan view of a unit hologram 24 that forms a part of the transmission type hologram 21. As shown, nine element holograms 241, each comprising a transmission type Fourier transform hologram, are lined up parallel with one another into the unit hologram 24, and the transmission type hologram 21 is assembled by arranging a multiplicity of such unit holograms 24 in rows and columns. FIG. 14(c) is illustrative of one exemplary reconstructed image 25 reconstructed upon viewing the point light source 23 through the transmission type hologram 21, which corresponds to the word “NOEL” in FIG. 16(b). In this case, the image has a crescent form.
As many images as the point light sources are reconstructed through the transmission hologram 21 comprising such unit holograms 24 as shown in FIG. 14(b) (=FIG. 15(b)). As nine point light sources 231 to 239 located behind the transmission hologram 21 are viewed through the transmission hologram 21 as shown typically in FIG. 15(a), it allows an image with nine identical (crescent) images lined up in rows and columns to be reconstructed and seen.
On the other hand, Patent Publication 3 proposes that a plurality of unit holograms 24 for reconstructing mutually different images are parallel arranged into a transmission type hologram, and the position of an eye with respect to that transmission type hologram is so varied that a single image differing depending on the light sources behind that transmission type hologram is reconstructed.    Patent Publication 1    U.S. Pat. No. 5,546,198    Patent Publication 2    JP(A)2004-126535    Patent Publication 3    JP(A)10-282870
However, such transmission type holograms as briefed above are shy of variations and interest, because, as long as the relative position with respect to the eye is fixed, as many single identical images as point light sources are only reconstructed.